(1) Field of the Invention
The present invention relates to a multi-layer plastic molded container. More particularly, the present invention relates to a multi-layer plastic molded container which is excellent in the barrier property to gases such as nitrogen, carbon dioxide gas, oxygen and steam, especially in the combination of the oxygen barrier property and moisture resistance, and in which the humidity dependency of the oxygen barrier property is prominently reduced and the properties of the barrier layer, such as shock resistance, scratch resistance, peel strength, weatherability, hot water resistance and content resistance are highly improved.
(2) Description of the Prior Art
Plastic bottles prepared by melt-extruding and hollow-molding (blow-molding) thermoplastic plastics such as polyolefins are used in various fields instead of glass bottles because the plastic bottles have a lighter weight and a better shock resistance than the glass bottles.
General-purpose plastics such as polyolefins are excellent in the moisture resistance and sanitary characteristics, but the oxygen permeability coefficient is relatively high and in bottles of these plastics, permeation of oxygen through bottle walls is not negligible. Accordingly, bottles of general-purpose plastics are not suitable as vessels for preserving foods for a long time or as vessels for cosmetics and the like where a high flavor-retaining property is required.
As bottles in which this defect is overcome, there have been developed and proposed plastic bottles having a wall structure excellent in the oxygen barrier property. Among melt-extrudable thermoplastic resins which are now available, a saponified ethylene/vinyl acetate copolymer (ethylene/vinyl alcohol copolymer) is most excellent in the oxygen barrier property. However, this saponified copolymer has inferior moisture resistance, that is, the steam barrier property, and in this saponified copolymer, the oxygen permeability coefficient tends to increase as the humidity increases. Accordingly, when this saponified copolymer is actually used for the formation of plastic bottles, it is necessary to adopt a troublesome molding method in which this saponified copolymer is sandwiched by moisture-resistant resins such as polyolefins and the resulting laminate is fed to the molding step to form a multi-layer laminate bottle. According to this method, however, the humidity dependency of the oxygen permeation coefficient of the saponified polymer is not completely eliminated, and increase in the oxygen permeation coefficient with increase in the humidity is especially prominent when an aqueous content is hot-filled in the above-mentioned laminate bottle or when the laminate bottle is subjected to retort sterilization.
We found that a copolymer comprising 99 to 70% by weight of vinylidene chloride and 1 to 30% by weight of at least one acrylic or methacrylic monomer and further including up to 100 parts by weight of other ethylenically unsaturated monomer per 100 parts by weight of the sum of the amounts of said two monomers (often called "vinylidene chloride copolymer" hereinafter) is a thermoplastic resin prominently excellent in the combination of the oxygen barrier property and the moisture resistance, as is seen from Table 1 given hereinafter, and the oxygen barrier property of this resin is not substantially changed according to the humidity of the measurement atmosphere, that is, the humidity dependency of the oxygen barrier properties is remarkably reduced. We have already proposed a bottle comprising this vinylidene chloride copolymer as a wall component and a process for the preparation thereof in our copending U.S. patent application Ser. No. 258,772.
The above-mentioned vinylidene chloride copolymer is still inadequate as the wall component of a plastic bottle because of the following defects.
(1) The melting temperature of the copolymer is very close to the thermal decomposition temperature thereof, and since thermal decomposition starts just after melting, melt extrusion molding of the copolymer is ordinarily difficult. PA1 (2) The impact strength is low and especially low at low temperatures, and the bottle wall of this copolymer is readily broken when the bottle is dropped or is shaken and the barrier property is lost. PA1 (3) Because of poor scratch resistance or weatherability, when the copolymer is used for coating a substrate bottle, the copolymer is readily deteriorated by shaking or outdoor exposure and peeled from the substrate bottle, resulting in loss of the barrier property. PA1 (4) Whitening is caused when the copolymer is dipped in hot water. Accordingly, if hot water is filled in a plastic bottle having the inner surface coated with the copolymer, the transparency of the bottle is reduced or lost. PA1 (5) Since the copolymer is soluble in an organic solvent such as tetrahydrofuran or cyclohexanone, a plastic bottle having the inner surface coated with the copolymer cannot be used for a content comprising such organic solvent as a base.